Perfect Corona & Babinet

A bright red fringed circular aureole surrounded by three, possibly four, rings. Imaged in February 2012 by Anders Falk Jensen while hiking in Torres del Paine National Park, Patagonia, Chile.

Image ©Anders Falk Jensen, shown with permission.

Coronae, Telescopes & Stars:

A corona is formed by tiny droplets in cloud or mist individually diffracting sunlight.

The light is scattered mainly at a droplet's surface and to a good approximation this can be thought of as an opaque disk rather than a sphere.

There is an optics principle, Babinet's Principle, that the diffraction pattern from an obstruction or mask is the same as that from its complement - in this case an opaque sheet with a circular aperture.

The latter configuration happens to be equivalent to a telescope aperture. A star seen under high magnification and steady atmospheric seeing is a small disk surrounded by 2-3 diffraction rings. A miniature corona where the scattering object is the telescope aperture.

Babinet's Principle is exact for diffraction patterns far from the scattering object, the far field or Fraunhofer diffraction, as here.

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